This invention relates to x-ray diagnostic systems and more particularly to a system which concentrates x-rays from a source and delivers them to an x-ray spectrometer.
Most focusing x-ray optics take advantage of total reflection at glancing angles of incidence. Total reflection occurs only when the angle of incidence is less than a critical angle that depends upon the properties of the reflecting material and the x-ray energy. Although prior art designs may vary according to application, most such designs have used metal or glass substrates with coatings of nickel, gold or iridium at glancing angles ranging from 10 to 150 arc minutes. Double-reflection geometries of the Wolter-I or Kirkpatrick-Baez types have been developed to focus a parallel beam of x-rays. The Wolter-I configuration consists of confocal parabaloid-hyperboloid shells and has been used most often for x-ray telescopes designed for high angular resolution. This optic is axially compact, has a moderate field of view and, in some cases, a large number of telescopes can be nested to fill a substantial fraction of the available entrance aperture. An approximation to the Wolter-I design replaces the precisely figured optics with simple cones. Telescopes based upon this approximation have been developed for various astrophysical payloads. The Kirkpatrick-Baez geometry uses two parabolic surfaces for parallel-to-point focusing, and it has been adapted to point-to-point geometries for x-ray microscopes. Recently, optics based upon bundles of glass capillary tubes have emerged as a method for focusing x-rays. The x-rays undergo numerous reflections as they travel through the glass channels causing these optics to have lower efficiency than the double reflection systems referred to above.
Electron microscopes are widely used in many applications including in the semiconductor fabrication industry. When targets are irradiated with electrons, x-rays are generated as a side effect. The x-ray spectrum provides information about elements contained in the target so that x-rays are often detected for analysis. In the prior art, it is known to place a detector such as a lithium-drifted silicon or germanium detector very close to the target in a scanning electron microscope. Such detectors are typically mounted on the end of a cold finger cooled by thermal conduction by means of a quantity of liquid nitrogen which boils at 77 kelvin. Higher resolution can be achieved utilizing detectors cooled to approximately 0.1 kelvin and in this context it may be desirable to locate the detector outside of the SEM enclosure. However, because of the well known square law dependence of intensity on distance from a source of x-rays, as a detector is moved farther from the source, the intensity drops which degrades the performance of a spectrometer receiving the x-rays. It is also known to use monolithic polycapillary glass optics within an SEM enclosure to concentrate x-rays for subsequent analysis but not to use any such concentrator beyond the confines of the SEM enclosure.
In one aspect, the x-ray diagnostic system of the invention includes a source of x-rays and an x-ray beam concentrator spaced apart from the x-ray source and disposed for receiving x-rays from the x-ray source. An x-ray spectrometer is disposed for receiving x-rays from the concentrator. The source of x-rays may be a point source such as a sample volume excited by an electron beam in a scanning electron microscope or excited by a focused synchrotron beam, an ion beam or a laser. The point source of x-rays may also be a commercial x-ray tube or may be produced by a small volume of hot gas produced in a laboratory plasma machine which may be of the magnetically and/or electrostatically confined type. The plasma can also be inertially confined. The x-ray source may also be a commercial electron impact device or even be a distant x-ray emitting object in space.
In preferred embodiments, the point-to-point x-ray concentrator is a single reflection concentrator made from either a nest of cylindrical surfaces or a surface wound into the form of a cylindrical spiral. The point-to-point concentrator may be a multiple reflection concentrator made either from opposed sets of nested conical surfaces or surfaces wound into the form of conical spirals. In another embodiment, the point-to-point concentrator is a single glass capillary bundle. The single glass capillary bundle may be monolithic. In another embodiment, the point-to-point concentrator includes a point-to-parallel glass capillary bundle coupled to a parallel-to-point glass capillary bundle and coupling occurs through vacuum or in gas over a variable distance.
It is preferred that the spectrometer be an energy dispersive x-ray detector such as a microcalorimeter, lithium-drifted silicon detector, germanium detector, cadmium zinc telluride (CZT) detector, gas scintillation proportional counter or gas proportional counter.
The spectrometer may also be a wavelength dispersive x-ray spectrometer which may use at least one flat Bragg crystal or may utilize at least one Bragg crystal in the Johann configuration or von Hamos configuration.
In yet another aspect, the invention is an x-ray concentrator comprising a ribbon of material having a reflecting surface and formed into a spiral having a plurality of windings. This concentrator may be either a single or a multiple reflection concentrator. It is preferred that the ribbon material be plastic foil, aluminum foil or quartz ribbon. A suitable plastic foil is polyester, kapton, melinex, hostaphan, apilcal or mylar. A particularly preferred plastic is available from the Eastman Kodak Company under the designation ESTAR(trademark). Suitable foil thicknesses range from 0.004 to 0.015 inches as required. It is preferred that the ribbon material be coated with a thin layer of metal, preferably a high Z metal such as nickel, gold or iridium. The metal coating may be multilayer. In a preferred embodiment, the spiral configuration is maintained by a support structure made of metal, plastic or a composite material. Suitable metals are aluminum, beryllium, stainless steel, titanium or tungsten.
In yet another aspect, the invention is an x-ray concentrator comprising a plurality of nested, concentric cylinders or cones made of a ribbon material having a reflecting surface. The nested cylinders or cones may be made of glass, aluminum foil, plastic foil, silicon or germanium. Suitable plastic material is the same as described above in conjunction with the spiral aspect of the invention. The plastic material would also be coated as described above in conjunction with the spiral configuration.
The concentrators of the invention may be located, for example, outside the enclosure of an SEM and receive x-rays through an x-ray permeable window or through an evacuated pipe with no window between the SEM and concentrator. The x-rays are concentrated or focused onto a spectrometer which may be located several meters from the target within the SEM. Because of the separation, spectrometers such as microcalorimeters cooled to on the order of 0.1 kelvin can be more conveniently utilized thereby giving much greater spectral resolution.